This invention relates, in general, to creped tissue paper products and processes. More specifically, it relates to dry creped tissue papers wherein an embryonic paper web is formed on a fourdrinier or similar paper making apparatus, adhesively secured while semi-dry to a cylindrical drying drum whereupon the drying of the web is substantially completed, and creped from the drum by means of a flexible creping blade. The invention also relates to novel, water dispersable polyamide-epihalohydrin creping adhesives complexed with metal ions.
Sanitary paper tissue products are widely used. Such items are commercially offered in formats tailored for a variety of uses such as facial tissues, toilet tissues and absorbent towels. The formats, i.e. basis weight, thickness, strength, sheet size, dispensing medium, etc. of these products often differ widely, but they are linked by the common process by which they originate, the so-called creped papermaking process.
Creping is a means of mechanically compacting paper in the machine direction. The result is an increase in basis weight (mass per unit area) as well as dramatic changes in many physical properties, particularly when measured in the machine direction. Creping is generally accomplished with a flexible blade, a so-called doctor blade, against a Yankee dryer in an on machine operation. This blade is also sometimes referred to as a creping blade or simply a creper.
A Yankee dryer is a large diameter, generally 8-20 foot drum which is designed to be pressurized with steam to provide a hot surface for completing the drying of papermaking webs at the end of the papermaking process. The paper web which is first formed on a formations forming carrier, such as a Fourdrinier wire, where it is freed of the copious water needed to disperse the fibrous slurry is usually transferred to a felt or fabric in a so-called press section where de-watering is continued either by mechanically compacting the paper or by some other de-watering method such as through-drying with hot air, before finally being transferred in the semi-dry condition to the surface of the Yankee for the drying to be completed.
The impact of the adhered web with the doctor blade is essential to impart to the paper web the properties which are sought by manufacturers. Of particular importance are softness, strength and bulk.
Softness is the tactile sensation perceived by the consumer as he/she holds a particular product, rubs it across his/her skin, or crumples it within his/her hand. This tactile sensation is provided by a combination of several physical properties. One of the most important physical properties related to softness is generally considered by those skilled in the art to be the stiffness of the paper web from which the product is made. Stiffness, in turn, is usually considered to be directly dependent on the strength of the web.
Strength is the ability of the product, and its constituent webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions.
Bulk, as used herein, refers to the inverse of the density of a tissue paper web. It is another important part of real and perceived performance of tissue paper webs. Enhancements in bulk generally add to the cloth like, absorbent perception. A portion of the bulk of a tissue paper web is imparted by creping.
The level of adhesion of the papermaking web to the dryer is also of vital importance as it relates to the control of the web in its travel in the space between the creping blade and the winder upon which a roll of the paper is being formed. In addition, different creped products require different levels of adhesion, tack and rewetability, for example a facial tissue web will require a different level of adhesion, tack and rewetability compared to a paper towel web. Webs which are insufficiently adhered tend to cause poor control of the sheet with consequent difficulties in forming a uniform reel of paper. A loose sheet between the creper and the reel will result in wrinkles, foldovers, or weaving of the edges of the sheet in the rolled-up paper. Poorly formed rolls not only affect the reliability of the papermaking operation, but also the subsequent operations of tissue and towel manufacture in which the rolls are converted into the tissue and towel products.
The level of adhesion of the papermaking web to the dryer is also of vital importance as it relates to the drying of the web. Higher levels of adhesion reduce the impedance of heat transfer and cause the web to dry faster, enabling more energy efficient, higher speed operation.
However, the level of adhesion is not the sole factor determining product quality and manufacturing reliability. For example, some adhesives have been found to form a bond between the web and the doctor blade at the point of creping such that the web does not dislodge properly so that portions of the web remain adhered to the dryer and travel past the edge of the blade. This causes a defect in the web and often causes the web to break.
Further, while some amount of build-up of the adhesive on the dryer is essential, excessive build-up or streaks can be formed with some types of adhesives. Streaks can cause differences in the profile of adhesion across the width of the dryer. This can result in humps or wrinkles in the finished roll of paper. A second doctor blade is often positioned after the creping blade in order to remove any excess creping adhesive and other residue left behind. This blade is referred to as a cleaning blade. Cleaning blades and creping blades must be changed at some frequency to prevent a streaky coating and loss of sheet control.
The term xe2x80x9cdoctorabilityxe2x80x9d as used herein refers to the relative ease with which the web is dislodged from the dryer without producing defects or requiring frequent changes of blades to prevent excessive build-up.
One important characteristic of a creping adhesive is that it be rewetable. xe2x80x9cRewetabilityxe2x80x9d, as used herein, refers to the ability of the adhesive film remaining on the heated drying surface to be activated by the moisture contained in the semi-dry issue web when the web is brought into contact with the heated drying surface. A marked increase in tack is indicative of high rewetability. Rewetability is important because only a portion of the drying surface is normally covered with adhesive on a given rotation of the Yankee dryer.
The majority of the adhesion of the sheet to the dryer occurs by means of the creping adhesive deposited in previous passes.
There is a natural tendency of paper making web to adhere to the cylindrical dryer owing to the build-up of deposits of both organic and inorganic components from the paper web. These components (fines, fillers and papermaking chemical additives) can form deposits that can impact the creping process efficiency at the point of transfer of the web to the cylindrical drum. The needs for specific level and type of adhesion however has induced considerable activity among researchers in the field. Consequently, a wide variety of creping adhesives are known in the art. The use of animal glue, for example, has long been known.
To maintain a specific and constant level of adhesion to provide a creped product with the desired physical properties, it is sometimes necessary to add a release agent with the adhesive to control or adjust the level of adhesion on the Yankee dryer surface.
Soerens, in U.S. Pat. No. 4,501,640, incorporated herein by reference, discloses an adhesive which comprises an aqueous admixture of polyvinyl alcohol and a water-soluble, thermosetting, cationic polyamide-epichlorohydrin resin.
Hollenberg et al., in U.S. Pat. No. 5,246,544, incorporated herein by reference, discloses an adhesive which comprises a water soluble hydroxylated polymer or oligimer and a cross linking metal cation.
In another example, Vinson et al., in U.S. Pat. No. 5,944,954, incorporated herein by reference, discloses a water soluble creping adhesive comprising a cationic starch and optionally a polyvinyl alcohol and a water-soluble, thermosetting cationic polyamide-epichlorohydrin resin.
While a number of adhesives including these examples have been disclosed and are available, no single adhesive or adhesive blend has provided a satisfactory combination of doctorability, rewetability, and level of adhesion.
In addition, a serious problem with the water-soluble, thermosetting cationic polyamide-epichlorohydrin resins is that the physical properties of the resin such as doctorability, rewetability and the level of adhesion are in large measure controlled by the degree of cross-linking by the epichlorohydrin so that it is very difficult to vary these properties for a given creping machine. Paper makers have recognized that the different creping machines and different wood pulps have different properties with the contemporaneous need for changing the properties of the creping adhesive to optimize the creping process. Paper makers have long wanted the ability to control and change the amount of cross-linking to achieve the precise amount of doctorability, rewetability and adhesion level desired for a given papermaking line to optimize the creping process. With the water-soluble, thermosetting cationic polyamide-epichlorohydrin resins of the prior art that degree of control has not been possible because the cross-linking of the adhesive occurs during the manufacturing process and it is impractical to have a number of water-soluble, thermosetting cationic polyamide-epichlorohydrin resins with different degrees of cross-linking in an attempt to tailor the creping adhesive to the creping process.
Therefore, it is an object of the present invention to provide an adhesive for creping tissue paper and a process for applying it which overcome these limitations by offering a creping adhesive that can be readily varied in a controlled manner so that the doctorability, rewetability and adhesion level desired for a given papermaking line can be optimized for a given creping process.
It is another object of the invention to provide a adhesive whose properties may be readily varied and adjusted by varying the percentage of the metal ion in the adhesive composition.
It is a further object of the invention to provide an improved creping process for the production of a creped paper product.
These and other objects are obtained using the present invention as will be taught in the following disclosure.
The invention is an aqueous dispersion useful as a creping adhesive comprising a water-dispersable thermally cross-linked polyamide-epihalohydrin resin with at least one multivalent metal ion. The weight percentage of metal ion to weight percentage of polyamide-epihalohydrin can vary from 0.05 to 12.0 weight percent, preferably 0.50 to 10.0 weight percent and even more preferably 0.75 to 8.0 weight percent.
The dispersion contains from about 90% to 99.9% water and more preferably, from about 95 to 99.9% water.
The polyamide-epihalohydrin resins preferably comprises the reaction product of an epihalohydrin and a polyamide containing secondary or tertiary amine groups. The epihalohydrin is preferably epichlorohydrin and the polyamide amine groups are preferably secondary amine groups derived from a polyalkylene polyamide and a saturated aliphatic dibasic carboxylic acid. The dibasic carboxylic acid preferably contains from about 3 to about 10 carbon atoms.
The mole ratio of epihalohydrin to secondary amine groups in the polyamide is preferably about 0.5 to 1 to about 2 to 1.
The multivalent metal ion is selected from calcium, strontium, barium, titanium, chromium, manganese, iron, cobalt, nickel, zinc, molybdenum, tin, antimony, niobium, vanadium, tungsten, hafnium and zirconium. Mixtures of metals ions can also be used. Preferred anions include acetate, formate, hydroxide, oxo, carbonate, chloride, bromide, and iodide.
In a preferred formulation the polyamide-epihalohydrin is cross-linked with the epihalohydrin to a given degree of cross-linking. A given metal ion or combination of metal ions is then selected and the metal ion is then added to coordinate with ligands located on the epihalohydrin-polyamide resin. The metal ion selected and the concentration of metal ion can be adjusted to vary the properties of the resin such as insolubility, rewetability, density, cross-linking, brittleness and to reduce or increase the tack or adhesion properties of the mixture. In this manner the properties of the creping adhesive can be readily adjusted so that the creping adhesive is optimized for a given drier and wood pulp composition. In general, for a constant degree of cross-linking by the epihalohydrin, as the concentration of metal ion increases the insolubility, rewetability, density, and brittleness increase and the degree of adhesion and tack decreases.
The invention further provides a process for creping tissue paper. The process comprises:
a. applying to a rotating creping cylinder an aqueous dispersion comprising from about 90% to about 99.9% water and from about 10% to about 0.1% solids, wherein said solids comprise a water dispersable, polyamide epihalohydrin and a multivalent metal ion;
b. pressing a tissue paper web against the creping cylinder to effect adhesion of the web to the surface of the cylinder; and
c. dislodging the web from the creping cylinder by contact with a doctor blade.
The total amount of applied creping adhesive is preferably from about 0.1 lb/ton to about 10 lb/ton based on the dry weight of the creping adhesive and the dry weight of the paper web. The unit lb/ton, as used herein, refers to the dry amount of creping adhesive measured in lbs. relative to the dry amount of paper measured in tons.
The tissue web can be comprised of various types of natural and recycled fibers including wood pulps of chemical and mechanical types. The fibers can comprise hardwood, softwood and cotton fibers. The tissue web can also contain particulate fillers, fines, ash, organic contaminates such as the cellophane from envelope windows, adhesives such as PVA-styrene-butadiene and release adhesives from Post-It notes and inks as well as process chemicals used in the paper-making process such as strength additives, softeners, surfactants and organic polymers.
In its preferred embodiment, the method is used to prepare tissue paper with a basis weight between about 10 g/m2 and about 50 g/m2 and, more preferably, between about 10 g/m2 and about 30 g/m2. The preferred density is between about 0.03 g/m3 and about 0.6 g/cm3, and more preferably, between about 0.05 g/cm3 and 0.2 g/cm3.
All percentages, ratios and proportions herein are by weight unless otherwise specified.